Conventionally, underwater communications' are achieved using acoustic method. This is because the conductivity of seawater is exceedingly high for practical implementation of underwater communications using electromagnetic radiation methods.
Specifically, the high conductivity in seawater causes large attenuation in electromagnetic radiation. This results in the electromagnetic radiation incapable of propagating over long distances.
Although the acoustic method is suitable for long distance underwater communications, the bandwidth of such communications is undesirably limited. Conventional acoustic modem is capable of communicating at a rate of 40 kbps (Kilobytes per second) for up to a distance of a few hundred meters (m). The rate decreases to approximately 10 kbps for a distance greater than 5 kilometers (Km). Additionally, acoustic underwater communications is affected when it occurs close to shore or when there is noise generated by physical movements from underwater objects that are near the acoustic modem.
With the advent of Autonomous Unmanned Vehicle (AUV), there is an alternative way of transmitting large amount of the data collected via underwater sensors. Instead of sending the data over long distances, AUV is used to reach the proximity of the underwater sensors (in the order of 10 m) to collect data from the sensors. A modem with data transfer rate that is much higher than the 40 kbps achieved by the conventional acoustic modem is desirable. Hence, there is a need for an alternative modem that is capable of delivering high bit rate over a short range in an underwater environment.
Previous attempts have been made to study underwater communications by means of electromagnetic radiation. Theoretical and experimental studies of dipole antennas immersed in seawater have been proposed by M. Siegel and R. W. P. King in “Electromagnetic Propagation Between Antennas Submerged in the Ocean,” IEEE Trans. Antennas Propagat., vol. 21, pp. 507-513, July 1973. However, the received signal level is undesirably low for existing communication systems.
This is especially so for existing narrowband systems as the bit rates that the systems are capable of supporting are unclear. A similar concept using a loop antenna is proposed by A. I. Al-Shamma'a, A. Shaw, and S. Saman in “Propagation of Electromagnetic Waves at MHz Frequencies Through Seawater,” IEEE Trans. Antennas Propagat., vol. 52, pp. 2843-2849, Nov. 2004. The authors have proposed that the attenuation in far field propagation is much smaller due to the existence of displacement current. However, this is not verifiable, as suggested by R. Somaraju and J. Trumpf in “Electromagnetic Wave Propagation and the Permittivity of Seawater”.
A method for underwater communication using electric current has been proposed by H. Momma and T. Tsuchiya in “Underwater Communication by Electric Current” IEEE OCEANS'76, pp. 24C1-24C6. This method is an alternative to the acoustic method for short-range underwater communications. The method is not affected by acoustic noise existing in underwater environment and has been shown to deliver data up to a distance of 150 m. However, the method results in high power consumption.
There is therefore a need to provide an alternative way for underwater communication that is power efficient and having an improved data transfer rate and communication range.